A display device which is very thin and lightweight as compared to the conventional cathode-ray tube display device, a so-called flat panel display, has been developed. A liquid crystal display device having a liquid crystal element as a display element, a display device having a self-luminous element, an FED (field emission display) using an electron source, and the like compete in the market of flat panel displays. Therefore, lower power consumption and a higher contrast are required in order to increase the added value and differentiate from other products.
A general liquid crystal display device is provided with one polarizing plate over each of the substrates so as to improve a contrast ratio. By darkening black display, the contrast ratio can be improved. Therefore, higher display quality can be provided when an image is seen in a dark room such as a home theater room.
For example, it is suggested that a first polarizing plate is provided on the outer side of a substrate on a viewing side of a liquid crystal cell, a second polarizing plate is provided on the outer side of another substrate facing the substrate on the viewing side, and a third polarizing plate is provided for heightening the polarization degree when light from an auxiliary light source provided on the substrate side is polarized through the second polarizing plate and passes the liquid crystal cell, in order to improve a contrast ratio (see Patent Document 1: PCT International Publication No. 00/34821). As a result, improvements in nonuniformity of display caused by an insufficient polarization degree and polarization degree distribution of the polarizing plate, and in the contrast ratio are realized.
As a flat panel display similar to a liquid crystal display device, there is a display device having an electroluminescence element. The electroluminescence element is a self-luminous element which requires no light irradiation means such as a backlight; therefore, a display device using it can be made thin. In addition, compared to a liquid crystal display device, the display device using the electroluminescence element has merits such as high response speed and less view angle dependence.
A structure in which a polarizing plate or a circularly polarizing plate is provided for a display device having such an electroluminescence element is proposed (see Patent Documents 2 and 3: Japanese Patent No. 2761453 and Japanese Patent No. 3174367).
Furthermore, as a structure of a display device having an electroluminescence element, a structure in which light emitted from a light emitting element interposed between light-transmitting substrates can be observed as light of an anode substrate side and light of a cathode substrate side is proposed (see Patent Document 4: Japanese Published Patent Application No. H10-255976).